1. Technical Field
The invention relates to a magnetic tape cassette, for example a tape recorder or video tape cassette, which is provided with an indexing device for the unwinding of the magnetic tape.
2. Prior Art
Such cassettes are practical to use but one is very poorly informed at a given moment as to the part of the tape in the course of recording or reading. Rough information is drawn from the estimation of the length of tape which remains to be wound or unwound and that is by viewing through a window provided in the case of the cassette.
A less inaccurate means consists in providing the reading and/or recording apparatus with a counter connected to the drive mechanism for the tape. Still, for the indication of the counter to have a significance, on the one hand, the tape must be unwound from its origin, on the other hand, the counter must be reset at the start of unwinding. In addition, the drive mechanism and/or the connection with the counter customarily comprises a belt drive so that slipping is inevitable on starting and stopping and the indication of the counter is in practice inaccurate and unreliable.
Indexing devices for the unwinding of the magnetic tape are also known which are integrated or secured to the cassettes themselves and which at least in theory are intended to give a very accurate indication of the unwound length of magnetic tape.
It has nevertheless been found in practice that these devices are too bulky to be integrated into a standard cassette, in particular a tape recorder cassette, or do not have a sufficient gear ratio to permit the accurate indexing of a given zone of the magnetic tape, or are formed of elements mounted so that the friction forces between these elements are very much greater than the traction force normally applied to the magnetic tape to unwind it, thereby risking the jamming of the cassette, the tearing of the tape, etc.